(running lots of sites in here)
I installed the plugin, no red error messages whatsoever from the backend.
Cache folder hadn't been created (~hotlink-cache), had to create it manually in blog root.
watermark.png was placed correctly in blog root.
.htaccess lines were added succesfully by the plugin also.
Went to add a new post, added a featured image and inserted the same image in post content.
Published the post. The cache folder it's empty.
Visited the post in the browser. Copied the link of the image and pasted into the browser to send no referrer. Nothing happened, I could see the original image.
What could it be?